1. Field of the Invention
The present invention relates to a method of fabricating a multi-domain liquid crystal cell, and, more particularly, to a simplified method of fabricating the wide viewing angle liquid crystal cell.
2. Discussion of Related Art
A twisted nematic liquid crystal display (TN LCD) has a contrast angular problem, i.e., the transmittance in each gray level depends on the viewing angle. This contrast angular dependence is especially strong in the up and down directions, and is caused by the electrically induced liquid crystal (LC) director configuration.
To solve this angular dependence problem, a multi-domain LCD such as a two-domain TN LCD (TDTN LCD) and a domain-divided TN LCD (DDTN LCD) have been introduced. In the TDTN LCD, each pixel has two director configuration domains, where the two pretilted directions are in opposing directions. Applying a gray level voltage to this LCD, the LC directors in two domains are tilted in opposite directions. These configurations average the up and down directions transmittance. In the DDTN LCD, materials having different pretilt angles, such as organic or inorganic materials, are alternately aligned in the each pixel. The aligning process results in each aligned area (i.e., each domain) having a pretilt angle different from that of the neighboring domain.
In the multi-domain liquid crystal cell discussed above, the most useful aligning method is the so-called rubbing method. In the rubbing method, the alignment layer, which consists of polyimide-coated layers, is mechanically rubbed with a rubbing cloth, etc., so that microgrooves are created on the surface of the alignment layer. The periodic topology of mechanically grooved LCD-substrates minimizes the elastic deformation energy of liquid crystals by forcing the director to align parallel to the microgrooves. In the rubbing method, however, the defect of the microgrooves causes random phase distortion and light scattering, so that the image quality deteriorates. Further, the rubbing process generates dust and discharge on the alignment layer causing the damage to the substrate and resultant yield deterioration.
A new method called the photo-alignment method was recently introduced in order to overcome the substrate damage problem.
FIGS. 1A-1E are views showing the fabrication method of the dual-domain (or two domain) cell using the photo-alignment process. In the figure, the hatched region of the substrate indicates the region blocked by the opaque mask, and the arrow in the substrate indicates alignment direction. The arrow above the substrate indicates the irradiation direction of the light.
First, the first domain I of the photo alignment material-coated substrate is blocked by the opaque mask. Then the substrate is exposed to vertical linearly polarized light having a first polarization direction, in order to define the first degenerated alignment direction in the second domain II, as shown in FIG. 1A. Subsequently, as shown in FIG. 1B, the substrate is exposed to oblique linearly polarized light having a second polarization direction which is perpendicular to the first polarization direction in order to select one direction of the first degenerated direction. As a result, the first alignment direction is formed in the second domain II.
Thereafter, the first domain I is uncovered, and the second domain II is covered with the mask. The substrate is exposed to vertical linearly polarized light having a third polarization direction perpendicular to the first polarization direction to define the second degenerated alignment direction, as shown in FIG. 1C. At this time, the degenerated second alignment direction is perpendicular to the first alignment direction. Subsequently, the substrate is exposed to oblique light in order to select one degenerated direction, as shown in FIG. 1D. FIG. 1E is a view showing the dual domain cell where the alignment directions of the domains are fully determined, after removing the mask. As shown in FIG. 1E, the alignment directions in the first and second domains are perpendicular to each other.
This process is again carried out for a second substrate, and then the two substrates are combined to form a dual-domain liquid crystal cell.
The alignment process of a multi-domain cell, such as a dual domain cell, however, is complex and costly, since eight exposure processes and four masking processes are needed.